Modern aircraft automatic flight control, pilot display and navigation subsystems require redundant air data and inertially derived data to perform functions such as aircraft stability augmentation, ride comfort enhancement, autothrottle control, control wheel steering, all-weather landing, long-range over water navigation, terminal area guidance and operation in advanced air traffic control areas. The inertial data necessary to perform these aircraft functions is presently furnished by body-mounted rate gyros, accelerometers, vertical gyros, directional gyros and platform inertial navigation systems (INSs). In many aircraft, INSs are not included because, while the inertial data available from platform INSs is highly desirable, the cost of producing such data cannot be justified by the function the aircraft is to perform. That is, platform INSs are expensive systems to produce and maintain. Because they are expensive, their costs must be justified based on the function an aircraft is to perform. If the function of the aircraft cannot justify the expense of procuring and maintaining a platform INS in view of available alternative systems, such as ground controller systems, an INS is not included. This result often occurs when the aircraft is only intended for operation over the landmass of a highly technological country, such as the United States.
As will be readily appreciated by those skilled in the inertial navigation art, a platform INS is expensive because it includes a number of gyros and accelerometers mounted close together on a small platform, which is rotationally decoupled from the structure of the aircraft by a set of at least three gimbal pivots. The gimbals allow the platform to be held in any preferred orientation (e.g., local level) independent of the angular motion of the vehicle. Not only do platform INSs have the disadvantage of being individually very expensive to produce and maintain but because of the redundancy requirements of many aircraft (such as commercial transport aircraft), not they have the further disadvantage that one but several expensive INS systems are normally required. As a result, the total INS costs are increased by the number of INSs required in a particular type of aircraft.
In view of the costs associated with platform INSs, proposals have been made to produce strapdown INSs (i.e., INSs wherein the sensing accelerometers and gyros are affixed to the vehicle) suitable for use in aircraft. However, while strapdown INSs have found widespread use in missiles, in the past for various reasons they have not been found acceptable for use onboard aircraft. In general, missile systems have been found to be unsuitable for use onboard aircraft because the navigation requirements of aircraft and missiles are substantially different. For example, aircraft INSs must remain reliable over long periods of continuous use, whereas missile INSs only require short lived precision. In this regard, INS errors are a direct function of time and distance. More specifically, INS errors are usually defined in terms of miles of position error per hour of travel. Because INS error is a function of time, an INS used on a vehicle (aircraft) that travels at a relatively low speed compared to other vehicles (missiles) must be substantially more accurate. As a result, strapdown INSs usable on missiles are generally unsuitable for use on aircraft.
Therefore, it is an object of this invention to provide a new and improved aircraft avionic system.
It is a further object of this invention to provide a strapdown INS suitable for use on an aircraft.
It is another object of this invention to provide a strapdown INS suitable for producing data similar to that produced by a platform INS and of equal or better accuracy and reliability.
It is also an object of this invention to provide an INS that includes a plurality of strapdown gyros and accelerometers whose sense axes are skewed to achieve higher effective redundancy and, thereby, higher reliability at lower cost.
Another difficulty with prior art avionic systems that include one or more INSs is that the INSs are not integrated with other aircraft sensing systems, such as air data sensors and temperature sensors, even though the information produced by these sensors is often used by the automatic flight control, pilot display and navigation systems of the aircraft in conjunction with the inertial information produced by INSs. It would be desirable to integrate these sensor systems together in order for the electronic subsystems (both interface and data processing) to be used in the most efficient manner so that overall costs are reduced.
Therefore, it is a still further object of this invention to provide an avionic system that integrates INS information with the information produced by other aircraft sensors.
It is yet a further object of this invention to provide a new and improved aircraft avionic system that includes a strapdown INS and integrates the information produced by the strapdown INS with information produced by other aircraft sensors in a manner such that a common set of sensors supply data to various aircraft subsystems and unnecessary duplication of sensors resulting from the inclusion of subsystem dedicated sensors is avoided.